|Environment and development
in coastal regions and in small islands
Coastal region and small island papers 3
Punta de Mangle, Isla de Margarita, Venezuela
Ramon Varela, Martin Llano, Juan Carlos Capelo, and Yully Velasquez
Estación de Investigaciones Marinas de Margarita (EDIMAR), Fundación La Salle de Ciencias Naturales, Apartado 144, Porlamar 6301-A, Isla de Margarita, Venezuela
The CARICOMP site at Isla de Margarita is a sandy spit covered by 16 ha of mangroves and 36 ha of seagrass beds. Coral reefs are not present at Punta de Mangle and are scarce around the island. Human activity at the site has been insignificant, historically, although there are harbor facilities nearby. The climate is semi-arid and strongly influenced by trade winds throughout the year (11-25 km h-1); the strongest winds occur February through June. The daily range of air temperature is equal to or greater than the seasonal variation (monthly average of 24.7-30.6°C). Total monthly precipitation is in the range 0-250 mm. Precipitation is most frequent in the months July-September and November-December. The driest months are March-May. Fringe-type mangroves are found along the coast: Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa. Studies on mangroves began in May 1992 with one plot; two more plots were established during 1995. Tree heights range from 7 to 12 m; trunk diameters range from 26 cm to 119 cm. The total litterfall analyzed over 3.5 years exhibit seasonality: maximum during May-July, 6.0 to 9.8 g m-2 d-1; minimum during November-February, 3.0 to 4.0 g m-2 d-1. Thalassia grows vigorously in most parts of the area. The biomass of the green parts ranges from 190 to 840 g m-2, with an increase in biomass during May. Production shows a large variation, from 2.88 to 11.03 g m-2 d-1; the greatest production coincides with the greatest part biomass recorded and with the times of flowering (May) and fruiting (July).
Punta de Mangle, which is located on the southern coast of Isla de Margarita (10°51'49"N, 64°03' 28"W; Fig. 1), is a sandy spit extending WSW from the island and oblique to the dominant direction of ocean currents, waves, and trade winds. The strait separating Isla de Margarita from Coche and Cubuagua Islands to the southeast and southwest, respectively, funnels and accelerates the predominantly E-W current, but this has little effect on the mangrove-fringed coastal zone. The major part of the energy is dissipated as the channel deepens into the Las Marites Depression and continues through the Margarita Depression to the Fosa de Cariaco (Gines, 1972). Punta de Mangle is the eastern limit of El Guamache Bay, on which are located several towns and a large dock for merchant ships.
|Fig. 1. Isla de Margarita, Venezuela,
showing the location of the CARICOMP
study site at Punta de Mangle.
The stations for study of the mangroves and seagrass meadows are located at the southern end of Punta de Mangle (Fig. 2). The entire area consists of 163 ha, in which is included 7 ha occupied by the harbor facilities at El Guamache, 26 ha of dry saline soils without vegetation, 36 ha covered by Thalassia beds, 21 ha of shallow muddy bottom covered with small algae, and 16 ha of mangroves. The mangroves are located exclusively at the southern half of the sandy point and are predominantly Rhizophora mangle and Avicenia germinans.
|Fig. 2. Punta de Mangle, showing its
bathymetry and the distribution of
mangroves and seagrass meadows. Depth contours are in meters.
Close to this site are two docks, one for merchant ships along the western face of Punta de Mangle, and the other an oil terminal situated 5 km to the east in Los Algodones. The coast is unpopulated between these docks; the closest human development is quite small and far from the coast. There is no evidence of detrimental effects upon the environment resulting from the presence of the docks. Conversely, the location is commonly visited by native fishermen who sweep the seagrass area with small seine nets. The area is also frequently visited by marine birds, which utilize this coastal area and the mangroves as a resting site.
Historical, Geological, and Human Inventory of the Study Site
Isla de Margarita is the most easterly part of the coastal mountain chain (Cordillera de la Costa) that covers the northern coastal region of Venezuela. This region is separated from the mountainous country of the interior by the Cariaco-Casanay tectonic depression (De Miró, 1974). Isla de Margarita is formed by two mountainous sectors oriented along an east-west axis and united by an isthmus. The eastern part is the more extensive and is Margarita proper, the western part is a peninsula called Macanao. The isthmus between the two sectors consists of extensive flat sedimentary areas where the principal coastal lagoons of the island are found. The southern coasts are characterized by the presence of shallow waters with abundant aquatic vegetation (mangroves and seagrasses) of considerable beauty (Hoyos, 1985).
The paleogeography of the study area is related to sea level fluctuations in the Caribbean and the tectonic morphology of the northern limit of the South American Plate (De Miró, 1974). Nevertheless, the geological history of Punta de Mangle is very recent; this sandy point probably developed during the late Holocene.
Human activity historically had little impact on the coastal ecology of Punta de Mangle, with the exception of the exploitation by local fishermen. The construction of the harbor facilities and access causeway at El Guamache in 1974 destroyed some mangrove areas in the small lagoons north of the study site, but this has seemingly not had a negative affect on the Punta de Mangle.
The mangroves on Isla de Margarita are now protected by environmental laws that forbid cutting mangrove areas for timber or development. However, the impact of public works (i.e., roads and piers) near the mangroves has not been taken into account, in particular runoff and the movement of the sub-soil waters and superficial sediments. This is the primary cause of the loss of mangrove areas on the island.
According to the classification of Köppen (Hoyos, 1985), the island is located in a zone of tropical humid climate under the subdivision "humid climate with short rainy seasons." Still, the climate is generally arid or semi-arid. Owing to the stationary high in the coastal zone during much of the year, rainfall is minimal, generating semi-arid conditions and characterized by a water deficit throughout the year. We present here a summary of 24 years of meteorological information collected in Punta de Piedras at the Fundación La Salle marine station, which is 6 km from the CARICOMP site study (Campo and Velásquez, 1991).
Between 1966 and 1989, temperatures fluctuated between 24.7 and 30.6°C (monthly average), during which there was a low frequency oscillation, with a period of 12 years overlying the annual variation. The highest monthly temperatures occurred between April and November, with September and October being the hottest. During that time, no temperature increase could be attributed to global warming. During the study period of January 1993 to October 1995, temperatures taken at Punta de Piedras showed a similar pattern (Fig. 3). What is significant is that the daily range is equal to or greater than the seasonal variation (mode of daily difference 6°C, maximum 12°C, minimum 1.5°C) . Again, no long-term trends are noticeable.
|Fig. 3. Daily air temperature and
precipitation recorded at Punta de Piedras
meteorological station, 1993-1995.
Historically, the average relative humidity has increased with time, with the most humid conditions recorded in January 1985 (91%) and the least humid in May 1967 (63%). For the study period 1993-1995, the average is 78%. The average monthly relative humidity is lowest between February and May and highest between June and November, with a plateau in October.
Total monthly precipitation ranges from zero to 250 mm, indicating the scarcity of rainfall in this area; the average monthly rainfall never exceeds 30 mm. The historical data indicate two groups of months in which precipitation is greatest: July-September and November-December (Campo and Velásquez, 1991). During these times, the precipitation averages 30-50 mm. The driest months are March through May, with an average rainfall of less than 12 mm. There is no permanent streamflow along the entire southern coast of Isla de Margarita, and rainwater runoff into Punta de Mangle region is insignificant because of the small drainage basin.
Daily observations from January 1993 to October 1995 demonstrate rainfall variability (Fig 3). There was significant rainfall in May through July in 1993 and 1994, but not in 1995; in 1994 and 1995, the second rainy season began earlier than in 1993.
Average daily evaporation is 9 mm; average monthly evaporation is approximately 266 mm. Comparing these values with precipitation values (<30 mm), it is clear that evaporative loss greatly exceeds precipitation gains. Average daily insolation is 8.5 hours, varying from zero to 12 hours. The months of greatest insolation are January to March, with about 9 hours; months with minimum insolation are June, July, and December, averaging 8 hours at the most. Average daily radiation varies between 250 and 600 cal cm-2 d-1 with an average 500 cal cm-2 d-1. During the 24-year period of data from Punta de Piedras, these values are similar to the average monthly temperature in that they show an oscillation associated with solar flares.
The average monthly wind velocity analyzed by Campo and Velásquez (1991) ranged from 11 to 25 km h-1, with the strongest winds occurring February through June. The wind pattern on Isla de Margarita is dominated by the trade winds, most commonly from the ENE, as indicated by data collected during 1975-1989.
For the past 200 years, hurricanes and tropical storms have had little effect on Isla de Margarita. Most such storms pass north of the island, producing torrential rains of short duration. The risk of hurricanes and severe meteorological impact is quite small (Carvajal and Buitrago, 1991).
Along the southern coast of Isla de Margarita, the zone of wave generation is reduced to a fringing stretch from east to west, owing to the easterly winds; waves from the north or northeast add little or nothing to the intensity of the local waves. Wave characteristics have not been determined locally; farther east, they vary from 0.6 to 1.0 m, with a periodicity of 4 to 7 sec. The waves at Punta de Mangle are smaller and with still shorter periodicity. Only during infrequent and anomalous atmospheric conditions would waves be generated from the south or southwest. Tides are typically semi-diurnal, with an average range of 0.22 m. The maximum water levels observed over nine years were a high tide of 0.6 m and a low tide of -0.42 m. In general, the marine current passing south of the island is a branch of the Caribbean Current that flows in a westerly direction close to the continental border of South America. This current is derived from the Guyana Current, which enters the Caribbean through the Grenada Strait located between the islands of Grenada and Trinidad and Tobago. Studies completed by the Estación de Investigaciones Marina de Margarita (EDIMAR) indicate that, at Punta de Mangle, the current flows east to west. However, a tide-associated countercurrent has been detected 10 m above the bottom (Campo, 1992). When the tide is rising, the direction of this current is to the west; when the tide is falling, the direction is to the east.
Vector analysis of average grain size was utilized to determine marine dynamics (Llano, 1989; Llano et al., 1991). Sediment type is a function of many variables, including energy forces. Based on preliminary studies of surface sediments from the Thalassia zone, we have determined that the site is affected by moderate kinetic energy, which dissipates toward the mangroves i.e., from south to north.
Three years of weekly temperature and salinity readings were taken at mangrove and seagrass sites (Fig. 4; CARICOMP Manual, 1991). The results indicate that the temperature of the water oscillates between 24.5°C and 32.0°C. Sea temperature is always somewhat higher than air temperature; it seems to follow fluctuations in minimum air temperature, as one can observe by comparing both temperatures. The water temperature minimum occurs in March-April, the maximum in August-September. The minimum coincides with the latter part of the upwelling season along the eastern coast of Venezuela (Muller-Karger et al., 1989). The salinity of the sea is relatively high, ranging from 35 to 40, values common for the southern coast of the island. Secchi depths show no clear tendency but may be dependent on local winds (Fig. 4).
|Fig. 4. Seawater temperature and
salinity at mangrove and seagrass
stations, Punta de Mangle, Isla de Margarita, along with Secchi disk
measurements of water transparency at the seagrass station.
Of the three communities studied in the CARICOMP program, only mangroves and seagrasses are found together and are well represented on Isla de Margarita. Coral reefs are scarce around the island and are not associated with seagrasses or mangroves. The paucity of coral reefs on the island may be related to the relatively cold water, the turbidity resulting from the occasionally high productivity of plankton, the proximity of discharge from the Orinoco River, and wind-induced turbulence, as well as normal currents. The definitive causes of the poor representation of hermatypic coral in the region have not been adequately studied. We are unaware of any biological or geological studies in the area of Punta de Mangle, or of any analysis of the marine ecosystems. Wagenaar Hummelinck (1977, 1981) mentioned Punta de Mangle as a site of both marine and land studies but provided no information about the flora and fauna. Goodbody (1984) utilized samples from Punta de Mangle for studies of ascidians.
The mangroves on Isla de Margarita are associated with hypersaline lagoons with no freshwater input. Typically, the trees are short, with the majority lacking a well-defined trunk. La Restinga and Las Marites are two coastal lagoons with extensive mangroves of this type. Other mangroves of limited range are found along the leeward coast, associated with small lagoons or growing in shallow and protected zones but always associated with seagrasses. Punta de Mangle represents this last type, which may be considered a mangrove fringe (Pannier and Fraiño, 1989).
The mangrove study area covers 16 ha. There is a small lagoon in the area, draining toward the southern coast. Among the three species of mangrove found in the study area, Rhizophora mangle (red mangrove) and Avicennia germinans (black mangrove) are well represented. The third species, Laguncularia racemosa, is not common but is represented by several small trees growing in dry sandy ground in the extreme west. Rhizophora forms a band 1,200 m long that is directly exposed to the sea. The community is well developed in comparison with nearby lagoon mangroves. Avicennia occupies a landward fringe without the continuity of Rhizophora, growing on saline ground affected by high tides.
Studies beginning in May 1992 recorded litterfall in a 100 m2 study plot populated exclusively by Rhizophora; these studies utilized the methodology proposed by CARICOMP (1991). There were five trees in this first study plot; tree heights ranged from 7 to 12 m; trunk diameters ranged from 26 to 119.3 cm, and basal area was 31.4 m-2 ha-1. Two additional plots were studied in 1995 but data are not reported here. The total litterfall in the study plot demonstrated a clear seasonal tendency (Fig. 5). Maximum litterfall occurred May-July: 9 g m-2 day-1 in June 1992; 9.8 g m-2 d-1 in July 1994; 7 g m-2 d-1 in May 1993; 6 g m-2 d-1 in May 1995. The minimum occurred in November through February, measuring 3-4 g m-2 d-1. Component analysis of the litter showed that flower biomass is constant during the year, indicating that a flowering season is not well defined. Although some seasonal effects occur, the production of leaves is well correlated with total litter. The seasonal fruit-seed falling from the trees is the element which most contributes to variations in the production of foliage.
Thalassia beds cover an area of 36 ha at Punta de Mangle. The distribution of these grasses seems to be related to type of bottom sediment. Thalassia is sparse in the area closest to the mangroves, which has a muddy soft bottom. The seagrass has poorly developed buds and is covered with many epiphytes and coated with fine sediments. The most abundant meadows are some distance from the mangroves (160 m; Fig. 2), where Thalassia has colonized the firmer sandy substrate and extends in a wide fringe along the coast. The depths at which Thalassia was found ranged from 0.2 m to more than 1.8 m, but the greatest depth could not be determined due to high turbidity levels. Thalassia grows vigorously at a depth of 0.4-1 m at the CARICOMP station. At a depth of 1.5 m, the leaves are very small and the shoots are less dense along the muddy bottom. From January 1993 through October 1995, the horizontal transparency of the water above the Thalassia beds ranged from 0.8 to 7.5 m, with an average visibility of 3 m (Fig. 4).
Preliminary biomass and productivity estimates have been made for the 18-month period between November 1992 and June 1994; it should be noted that, in some instances, insufficient replicates were taken during each sampling. The biomass of green parts ranged from 190 to 840 g m-2 (Fig. 6). Even considering the limitations of obtaining only one nucleus per collection, we recorded an increase in green biomass beginning in May. Non-photosynthetic tissues demonstrated a higher biomass: 1,420 g m-2 on average, with one large variation among the samples. There was no evidence of seasonality. The biomass of dead tissues also varies and showed no pattern; they averaged 1,380 g m-2.
|Fig. 6. Monthly variations of Thalassia
biomass (standing crop) in
Punta de Mangle, Isla de Margarita.
Thalassia production and daily turnover in 1993 and 1994 are shown in Table 1. Production oscillated between 2.88 and 11.03 g m-2 d-1. The greatest production coincides with the highest green biomass values as well as with the time of flowering in May and fruiting in July.
|Table 1. Thalassia production in Punta de Mangle, Isla de Margarita, Venezuela.|
|Date of Measurement||Production
(g m-2 d-1)
(% per day)
We wish to thank Dr. Craig Franz (La Salle University, Philadelphia) for his suggestions and help with English translation. The field work would not have been possible without the aid of Juan Martin Salazar, Jose Narvaez, and Fresdo Velásquez of the meteorological station at Punta de Piedras. EDIMAR Contribution No. 228 (Fundación LaSalle de Ciencias Naturales).
Campo, M. 1992. La Dinámica Marina entre la Isla de Margarita y La Península de Araya. Informe Final al Proyecto Conicit, Subvención: S1-1570. Fundación LaSalle, Estación de Investigaciones Marinas de Margarita, EDIMAR, 440 pp.
Campo, M., F. Velásquez. 1991. Resumen Climatológico de la Estación Meteorológica de Punta de Piedras, Estado Nueva Esparta, Venezuela. Período: 1966-1989. Pole de Recherche Oceanologique et Halieuthique Caraibe, Document Scientifique No. 29, Mai 1991; Ifremer, UAG, ORSTOM, 39Êpp., 39 fig.
CARICOMP. 1991. Manual of Methods for Mapping and Monitoring of Physical and Biological Parameters in the Coastal Zone of the Caribbean. UNESCO, Paris, 35 p.
Carvajal, F., J. Buitrago. 1991. La Ostra Perla Pinctata Radiata Leach, en el Nororiente de Venezuela y su Posibilidad de Cultivo. Informe técnico EDIMAR, 42 pp.
De Miró, M. 1974. Morfológia Submarina y Sedimentos Marinos Recientes del Margen Continental del Nororiente de Venezuela. Cuadernos Azules No. 14, Caracas, 230 pp.
Gines, H. 1972. Carta Pesquera de Venezuela: 1, Area del Nororiente y Guayana. Fundación La Salle de Ciencias Naturales, Monografía No. 16, Caracas, 328 pp.
Goodbody, I. 1984. Ascidians from Caribbean shallow water localities. Studies on the Fauna of Curaçao and Other Caribbean Islands, 67: 62-76.
Hoyos, J. 1985. Flora de la Isla de Margarita, Venezuela. Sociedad y Fundación La Salle de Ciencias Naturales, Monografía No. 34, Caracas. 927 pp.
Muller-Karger, F. E., C. R. McClain, T. R. Fisher, W. E. Esaias, R. Varela. 1989. Pigment distribution in the Caribbean Sea: Observations from space. Progress in Oceanography, 23:23-64.
Llano, M. 1989. Dinámica sedimentaria en la Bahía del Guamache, deducida del análisis vectorial del tamaño medio de los granos. Boletín Instituto Oceanográfico de Venezuela, Universidad Oriente, 28 (1-2):269-275, 1 table, 6 fig.
Llano, M., P. I. Guevara, A. Acevedo. 1991. El análisis vectorial en la determinación de zonas de erosión, transporte y sedimentación en lagunas costeras. Memoria Sociedad Ciencias Naturales La Salle, 51 (135-136):43-56.
Pannier, F., R. Fraiño de Pannier. 1989. Manglares de Venezuela. Cuadernos Lagoven, Caracas, 67 p.
Wagenaar Hummelinck, P. 1977. Marine localities. Studies on the Fauna of Curaçao and Other Caribbean Islands, 51:15-68.
Wagenaar Hummelinck, P. 1981. Land and freshwater localities. Studies on the Fauna of Curaçao and Other Caribbean Islands, 63:1-133.